1. Field of the Invention
This invention relates to a steering system for a vehicle which turns wheels of the vehicle in response to operation of the steering wheel, and more particularly to a steering system for a vehicle having a hydraulic power cylinder which operates under the control of a hydraulic switching valve and turns wheels of the vehicle in response to operation of the steering wheel. The steering system of the present invention is useful, for example, for a system which turns the rear wheels in a four-wheel-steered vehicle.
2. Description of the Prior Art
As disclosed for instance, in U.S. Pat. No. 4,732,231, there has been known a four-wheel steering system in which the rear wheels are turned in response to operation of the steering wheel under the force of a hydraulic power cylinder which is controlled by a hydraulic switching valve and the rear wheel steering ratio which is the ratio of the turning angle of the rear wheels to the turning angle of the steering wheel is changed by a rear wheel steering ratio changing mechanism.
As shown in FIG. 24, in the four-wheel steering system disclosed in the above identified United States patent, a front wheel turning rod 2 is displaced in the transverse direction of the vehicle body in response to rotation of a steering wheel 1 and front wheels 3 are turned. The rotation of the steering wheel 1 is transmitted to a rear wheel steering ratio changing mechanism 5 by way of the front wheel turning rod 2 and a relay shaft 4.
The rear wheel steering ratio changing mechanism 5 has an output displacement member 5a which is displaced in the transverse direction of the vehicle body by a distance corresponding to the turning angle of the steering wheel 1. The ratio of the amount of displacement of the output displacement member 5a to the turning angle of the steering wheel 1 (This ratio corresponds to the ratio of the rear wheel turning angle to the steering wheel turning angle, and will be also referred to as "the rear wheel steering ratio".) can be changed according to the amount of rotation of a stepping motor 6. The amount of rotation of the stepping motor 6 is controlled by a control circuit 8 according to a vehicle speed signal output from a vehicle speed sensor 7. The actual amount of rotation of the stepping motor 6 is detected by a steering ratio sensor 9 and is controlled on the basis of the output of the sensor 9.
The steering wheel turning angle, the amount of displacement of the output displacement member 5a, the amount of rotation of the stepping motor 6 are values which represent not only the absolute value of the rotation or the displacement but also the direction of the rotation or the displacement, the direction of the rotation or the displacement being represented by plus or minus.
The displacement of the output displacement member 5a of the rear wheel steering ratio changing mechanism 5 is transmitted to a spool 10a of a hydraulic switching valve 10. The hydraulic switching valve 10 feeds a hydraulic pressure to a hydraulic power cylinder 11 in response to displacement of the output displacement member 5a so that a rear wheel turning rod 12 is displaced in the transverse direction of the vehicle body by an amount corresponding to the displacement of the output displacement member 5a, thereby turning rear wheels 13.
The spool 10a of the hydraulic switching valve 10 is accommodated in a valve housing 10b for displacement in the transverse direction of the vehicle body. The valve housing 10b fixed to the rear wheel turning rod 12 to be displaced in the transverse direction of the vehicle body together with the rear wheel turning rod 12. When the spool 10a is displaced, for instance, rightward from the neutral position shown in FIG. 24, a hydraulic pressure is introduced into a left hydraulic pressure chamber 11a of the power cylinder 11 by an oil pump 14 and a piston 11b fixed to the rear wheel turning rod 12 is displaced rightward under the hydraulic pressure in the left hydraulic pressure chamber 11a, whereby the rear wheel turning rod 12 is displaced rightward. The valve housing 10b is displaced rightward together with the rear wheel turning rod 12 and when the spool 10a substantially returns to the neutral position with respect to the valve housing 10b (more strictly to a balance position to be described later), introduction of the hydraulic pressure into the left hydraulic pressure chamber 11a is interrupted. When the spool 10a is further displaced rightward in this state, the rear wheel turning rod 12 is further displaced rightward by an amount corresponding to the amount of displacement of the spool 10a in the manner described above. When the spool 10a is displaced leftward, hydraulic pressure is introduced into a right hydraulic pressure chamber 11c and the rear wheel turning rod 12 is displaced leftward by an amount corresponding to the amount of the leftward displacement of the spool 10a.
However, the conventional steering system is disadvantageous in that the hydraulic switching valve part is large and heavy, thereby preventing reduction in size and weight of the overall steering system.
That is, in the hydraulic switching valve 10, a connecting portion 10c for fixing the valve housing 10b to the rear wheel turning rod 12 is formed integrally with the valve housing 10b, and the valve housing 10b is displaced together with the rear wheel turning rod 12 by the same amount as the rear wheel turning rod 12, e.g., about plus or minus 10 mm (i.e., about 10 mm leftward or rightward) from the neutral position shown in FIG. 24. The connecting portion 10c adds to the overall weight and size of the hydraulic switching valve 10, and as can be understood from the description above, a space for permitting the valve housing 10b including the connecting portion 10c to be displaced in the transverse direction of the vehicle body must be provided. Thus the size and weight of the overall steering system are increased.